1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a liquid crystal display (LCD) device having a structure for improving a viewing angle and a method for manufacturing the same.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device has upper and lower substrates spaced apart from and opposed each other, and a liquid crystal layer interposed therebetween. The upper and lower substrates respectively have an electrode thereon to form an electric field between the upper and lower substrates. The electric field formed between the upper and lower substrates aligns liquid crystal molecules so that an image can be displayed by a changing light transmittance according to an alignment of the liquid crystal molecules. Although there are many types of the liquid crystal display (LCD) devices, an active matrix liquid crystal display (LCD) device, in which film transistors and pixel electrodes connected to the thin film transistor are arranged in a matrix form, has been most widely used for the liquid crystal display (LCD) device because of its superior resolution and moving image display ability. The aforementioned liquid crystal display (LCD) device has a pixel electrode on the lower substrate and a common electrode on the upper substrate and drives liquid crystal molecules by an electric field formed perpendicular to the upper and lower substrates. Many types of liquid crystal materials are used for the liquid crystal display (LCD) device, and among them, a twisted nematic (TN) mode liquid crystal is most widely used for the liquid crystal display (LCD) device. The TN mode liquid crystal has an alignment structure of the liquid crystal molecules in which every liquid crystal molecule is aligned parallel to the upper and lower substrates and the liquid crystal molecule close to the upper substrate and the molecule close to the lower substrate are twisted 90° (degree) with respect to the upper and lower substrates when voltage is not applied to the liquid crystal molecules. In addition, the liquid crystal display (LCD) device usually has a single domain structure of the liquid crystal in which the liquid crystal molecules for each a sub-pixel has the same alignment characteristics.
FIG. 1A is a plan view of a related art TN (twisted nematic) mode liquid crystal display (LCD) device having a single domain structure, and FIG. 1B is a cross-sectional view taken along a line “I—I” in FIG. 1A. In FIG. 1A, a gate line 12 is formed in a first direction and a data line 22 is formed in a second direction. The gate and data lines 12 and 22 define a pixel region “P” by crossing each other, and a thin film transistor is formed near a crossing portion of the gate and data lines 12 and 22. A pixel electrode 26 connected to the thin film transistor is formed in the pixel region “P”. A solid arrow line and a dotted arrow line in the pixel region “P” respectively illustrates rubbing directions of upper and lower alignment layers formed respectively on the upper and lower substrates. The upper and lower alignment layers are rubbed substantially perpendicularly to each other and the pixel region “P” is not divided. (I.e., there is a single domain of liquid crystal alignment direction when an electric field is applied). In FIG. 1B, the first and second substrates 30 and 50 are spaced apart from and opposed each other. A thin film transistor “T” and a pixel electrode 26 connected to the thin film transistor “T” are formed on the first substrate 30. A first alignment layer 28 is formed on the first substrate 30 on which the thin film transistor “T” and the pixel electrode 26 are already formed. A black matrix 52 corresponding to the thin film transistor “T” is formed on an inner surface of the second substrate 50 and a color filter 54 is formed on the black matrix 52 and second substrate 50. A common electrode 56 is formed on the color filter 54 and a second alignment layer 58 is formed on the common electrode 56. A layer 70 of TN (twisted nematic) mode liquid crystal is disposed between the first and second substrates 30 and 50. As discussed previously because the first alignment layer 28 is rubbed in a perpendicular direction to a rubbing direction of the second alignment layer 58, the TN mode liquid crystal has a 90° (degree) twisted structure when voltage is not applied to the pixel and common electrodes 26 and 56. As illustrated in FIG. 1B, the liquid crystal molecules 72 are aligned perpendicular to the first and second substrates 30 and 50 when the voltage is applied to the pixel and common electrodes 26 and 56. Thus the liquid crystal layer 70 serves as a light shutter for a light source (not shown). In the liquid crystal display (LCD) device having the TN mode liquid crystal, there is a difference between light intensity L1 controlled by a long axis of the liquid crystal molecules 72 and light intensity L2 controlled by a short axis of the liquid crystal molecules 72 as a viewing angle of an observer changes. Accordingly, a viewing angle property of the liquid crystal display (LCD) device is not good and thus the observer perceives that a luminance of the liquid crystal display (LCD) device is not uniform. To overcome aforementioned problem, a liquid crystal display (LCD) device having a multi-domain structure in which a sub-pixel or a pixel is divided to align the liquid crystal molecules symmetrically. In general, a multi-domain structure for the TN mode liquid crystal display (LCD) device is formed by controlling a rubbing direction of the alignment layer or forming an abnormal electric field. According to the latter method for forming the multi-domain structure, an alignment of the liquid crystal molecules can be stabilized in the multi-domain structure by forming a fringe field by the abnormal electric field.
FIG. 2A is a plan view of a related art TN (twisted nematic) mode liquid crystal display (LCD) device having a two-domain structure, and FIG. 2B is a cross-sectional view taken along a line “II—II” in FIG. 2A. In FIG. 2A, two domains are formed in a pixel region “P” by controlling a rubbing direction and explanations about the same elements as those in FIG. 1A will be omitted for the sake of explanation. In FIG. 2A, gate and data lines 112 and 122 define a pixel region “P” by crossing each other, and first and second domains “IIIa” and “IIIb” are formed in the pixel region “P”. The two-domain structure can be formed by controlling the rubbing direction of the first and second alignment layers (not shown) as aforementioned. In FIG. 2B, the rubbing direction of the first and second alignment layers 128 and 158 in the first domain “IIIa” and the rubbing direction of the first and second alignment layers 128 and 158 in the second domain “IIIb” are symmetric about a boundary between first and second domains “IIIa” and “IIIb,” as illustrated in FIG. 2A. Accordingly, liquid crystal molecules 172 in the first domain “IIIa” and liquid crystal molecules in the second domain “IIIb” are aligned symmetrically to each other about the boundary between the first and second domains “IIIa” and “IIIb”. As a result, a light intensity L11 controlled by a long axis of the liquid crystal molecule in the first domain “IIIa” and a light intensity L22 controlled by a long axis of the liquid crystal molecule in the second domain “IIIb” becomes similar to each other and so that a viewing angle can be improved. However, in the aforementioned method for dividing the sub-pixel or the pixel to have the multi-domain structure by controlling the rubbing direction of the alignment layer, properties of the divided domains depend on a rubbing property of the alignment layer. Accordingly, a minor defect in rubbing may produce a defective liquid crystal display (LCD) device.